The invention relates to a method of manufacturing a picture display tube, in which method a gettering device is provided in the tube. The gettering device comprises a first metal holder containing a source of evaporable gettering metal, a second metal holder connected to an outer surface of the first metal holder and containing a gas source consisting of a material which releases gas upon heating. The tube is evacuated and then the gettering device is heated inductively to release the gas from the gas source and to evaporate the gettering metal from the source of gettering metal.
The invention furthermore relates to a picture display tube thus manufactured as well as to a gettering device suitable for use in the above-mentioned method.
Such a method is disclosed in U.S. Pat. No. 3,768,884. In the known method the first metal holder comprises a ring of an inductively heatable material, in which ring the gettering metal to be evaporated and a first gas source of gas-releasing material are incorporated. The second metal holder comprises a second gas source of gas-releasing material. These first and second metal holders are separated from each other so that during the inductive heating the temperature of the second holder lags behind with respect to that of the first holder. In this manner first the gas from the first gas source is released, then the gettering metal is evaporated and during this evaporation the gas from the second gas source is released. The object of this known method is to cause the scattering effect which the released gas exerts on the evaporating gettering metal, to take place over a longer period of time than would be the case when only one single gas source is used.
The known gettering device is suitable when using gas sources which give off their gas at comparatively low temperatures. A frequently used gas source belonging to this category is iron nitride (Fe.sub.4 N) which begins to decompose at approximately 500.degree. C. However, a number of restrictions are associated with the use of iron nitride, both with respect to the manufacture of the gettering device itself and with respect to the manufacture of the display tube in which said gettering device is to be used. For example, the low decomposition temperature of iron nitride restricts the maximum permissible temperature during degassing of the gettering device. Furthermore, iron nitride cannot withstand the action of moist air at approximately 450.degree. C. which conditions occur during the manufacture of a colour television display tube when the display window and the cone of the display tube are sealed together by means of a sealing glass. The use of iron nitride then does not permit the gettering device to be provided in the tube before the display window and the cone have been sealed together. This is a serious restriction, especially in the manufacture of colour display tubes having a resistive layer provided internally on a part of the tube wall, as described in British Patent Specification No. 1,226,728. This resistive layer is located near the neck-cone transition of the tube and this makes it necessary for the gettering device to be mounted in the tube in a place remote from the neck-cone transition to prevent the resistive layer from becoming electrically short-circuited by gettering metal evapour-deposited from the gettering device. Because of the inaccessibility of such a place, it is preferable to install the gettering device before the cone is sealed to the window of the tube. Such installation also eliminates the conventional practice of attaching the gettering device to the gun system assembled in the neck of the tube by means of a resilient metal strip, to avoid the forces exerted on the gun system by the metal strip.
A gas source which does not exhibit the above-mentioned restrictions with respect to iron nitride is disclosed in British Patent Specification No. 1,405,045. In this Specification the gas source comprises germanium nitride, Ge.sub.3 N.sub.4, as a gas-releasing material. Germanium nitride is a stable compound which can be exposed, without deterioration to moist air at a temperature of at least 450.degree. C. However, as compared with iron nitride, germanium nitride has a comparatively high decomposition temperature and gives off its nitrogen only during the evaporation of the gettering metal. In order to obtain a layer of gettering material on an inner surface of the tube, which is porous through-out its thickness and hence is readily absorbent, it is necessary that during the heating of the gettering device and before the gettering metal begins to evaporate, the gas released from the gas source has built up a sufficient gas pressure of approximately 133.times.10.sup.-3 to 666.times.10.sup.-2 Pa in the tube.